1. Field of the Invention
The present invention generally relates to an automatic watthour meter adjusting system. More specifically, the present invention is directed to an electronic type watthour measuring apparatus equipped with an automatic measuring-error correcting circuit, utilized in a watthour meter manufacturing factory, or an end user.
2. Description of the Prior Art
Conventionally, so-called "pulse-count comparison method" has been widely introduced to measure errors occurring in the electronic type watthour meters in the watthour meter manufacturing factories, or the end users. The basic idea of this conventional pulse-count comparison method will now be summarized.
As shown in FIG. 1, electrical power generated from a power generating source 3 is supplied not only to a watthour meter 1 which should be tested, but also to a master (reference) watthour meter 3 which has been previously adjusted or calibrated by another standard watthour meter (not shown) with a higher precision, or sensitivity than that of the tested watthour meter 1 as well as the master watthour meter 3. Then, after measuring this electrical power, the tested watthour meter 1 outputs count pulses "CP" in proportional to this measured power, whereas the master watthour meter 2 outputs reference pulses "RP". Both the count pulses "CP" derived from the tested watthour meter 1, and the reference pulses "RP" derived from the master watthour meter 3 are supplied to an RC counter 4. Precisely speaking, these pulses "CP" and "RP" are counted by a counter 41 for the master watthour meter 3, and a counter 42 for the tested watthour meter respectively during a predetermined time period. Accordingly, count values of these master meter counter 41 and tested meter counter 42 are displayed in the RC counter.
Subsequently, an operator manually calculates an error (namely, a measuring error of the meter 1) contained in the count value of the tested watthour meter 1 with respect to the count value of the master watthour meter 2 in accordance with the following formula (1): EQU E(%)=(CP value-Rp value).times.100/RP value (1),
where symbol "E" indicates an error in unit of percent, symbol "CP value" represents the count value of the tested meter counter 42, and symbol "RP value" denotes the count value of the master meter counter 41.
In accordance with such a conventional pulse-count comparison method, there is a merit that although precision of the master watthour meter 3 may cause measurement precision, precision of the power generating source 2 need not be so high, but also high stability thereof is not required, and therefore variations in the electrical power produced from this power generating source 2 are acceptable to some extent.
FIG. 2 represents an internal circuit arrangement of the tested watthour meter 2. In this internal circuit arrangement, both a current component "1" of electric power supplied from the power source 1 and a voltage component "V" thereof are detected by a current transformer (CT) 11 and a potential transformer (PT) 12, respectively, so that both a current signal and a voltage signal are derived from the CT 11 and the PT 12. The current signal derived from the CT 11 is directly supplied to a power/frequency converter 13, whereas the voltage signal derived from the PT 12 is supplied via a variable resistor 17 thereto. Then, the current signal is multiplied with the voltage signal in the power/frequency converter 13, whereby the multiplied value (namely, electrical power) is converted into a pulse signal, whose count value is directly proportional to the multiplied value. The pulse signal is frequency-divided into another pulse signal having such a frequency suitable for the pulse counting operation by a frequency divider 14. The frequency-divided pulse signal is processed in an output unit 18, thereby obtaining a processed pulse signal (namely, the above-described count pulse "CP") which will then be supplied to the RC counter 4 shown in FIG. 4. At the same time, this frequency-divided pulse signal is counted by a test meter counter 15, so that the count value (namely, watthours measured by this tested watthour meter 1) is displayed on a display unit 16.
To perform the adjustment of the tested watthour meter 1 with the above-described circuit arrangement in accordance with the conventional pulse-count comparison method, the following manual error-correcting operation must be carried out by an operator. That is, after the first error measurement (manual error calculation), an error is calculated from the count value corresponding to watthours displayed in the RC counter 4. Based on the last-mentioned error, the variable resistor 17 connected to the output terminal of the potential transformer 12 is manually adjusted so as to reduce this calculated error to be zero. Subsequently, a series of error measuring operation is again performed and a check is made whether or not a present error is involved within an allowable error range. If the present error is deviated from the allowable error range, the above-described error adjustment with controlling of the variable resistor 17 and also the above-explained error measurement are repeatedly performed until the measured error is involved within this allowable error range.
As previously explained in detail, in accordance with such a conventional error correcting method for the tested watthour meter, in order to reduce the error contained in watthours measured by the tested watthour meter 1 to the allowable value, the variable resistor 17 must be manually adjusted by an operator, or a worker in a factory. In other words, such a conventional error correcting operation requires plenty of manual adjustment procedure. As a consequence, a lengthy and cumbersome manual adjustment operation is necessarily needed, resulting in lowering of work efficiencies and expensive adjusting operation.